Does anyone know ...

how to remove a Canon FD to EF mount adaptor from the FD lens?

This is a Vivitar 28mm FD mount lens I bought from eBay to use with the Canon A1 film camera I have been playing experimenting  with.  It looks to be in good condition but the mount has an adaptor attached which fits a modern Canon EF lens mount e.g. my modern DSLR :-)

I'm assuming the adaptor looks like this and that  turning the ring toward open should release it.  Although it does feel looser, I can't see how it should come away from the lens. Also can't see any additional buttons to press, or any other obvious release mechanism on the lens.